In an offshore environment, well conductors are installed soon after the platform is secured in place so as to provide support for subsequent well casings or other drilling equipment which are inserted therethrough. Well conductors are normally large tubes having a diameter of about 20 inches or more and when installing these conductors, generally one of the following methods is used.
The first method (which has lost favor in the industry due to recent improvements in equipment) involves the welding of stops or padeyes to the outer surface of each conductor. These stops bear on framing members (which may be either permanent or temporary) that are designed to support the entire conductor string hanging from the top of the platform until the string becomes self-supporting. Consequently, the number of conductor strings which can be worked simultaneously is limited by the strength of these framing members and the overall ability of the platform to resist such loading.
During installation and as additional conductor lengths are needed, a crane is used to lift each individual conductor length from a supply barge, upend it, and vertically stab it in place. Afterwards, when the new length is securely added to the string, the crane lifts the entire string (a feat in itself-) so that the lower stops can be removed in order to lower the string the length of the new member. Stops secured to the upper end of the new member would then engage the framing members and the whole process would start over again. As can be imagined, this method is very slow and time consuming, it being costly in terms of labor, needed crane capacity, and crane time since the same crane that upends the new length must also lift the entire string, a separate smaller crane is unable to handle either procedure. The cost of fabricating stops and the cost associated with removing stops is often significant.
An improvement to this method involves specialized external and internal grippers that grab and hold the conductors in lieu of the aforementioned stops and/or padeyes. The internal gripper is generally secured to the crane while the external gripper is generally secured to the platform. In this fashion, the crane uses the internal gripper to hoist the conductor length and position it onto the conductor string for subsequent welding. The external gripper, which supports the string during this operation, is deactivated only when it is desired to allow the new conductor length to slide through it (the crane supporting the entire conductor string during this operation). While this method is quicker in that there is no need to continuously add and then remove stops and/or padeyes, it still requires the addition of framing members to the platform so as to support the string until it becomes self-supporting and it still requires a very large crane for both upending the new member and for lowering the entire conductor string.
In order to reduce the needed crane time, a system has been developed utilizing two external grippers, one being movable with respect to the other by a series of jacks. With this system, a large crane and the internal gripper or padeyes and slings would still be used to lift the new conductor length and align it with the string for welding as before. Afterwards, however, one external gripper (which is in a raised position) would be lowered by the jacks toward the other external gripper, this lower gripper being deactivated so as to allow the string to slip through it. In this fashion, the entire conductor string is always supported by one or both of the external grippers and not by the crane. While this method eliminates the need for large crane tonnage, it is a very slow process due to the leisurely pace and small stroke of the jacks. Additionally, framing members are still needed and the platform itself must still be designed so as to withstand the temporary imposition of large installation loads.
It is thus an object of this invention to provide a method of installing conductors in an offshore environment that reduces the need for large crane tonnages. Another object of this invention is to provide a method that substantially eliminates the need for additional framing members. Still another object of the invention is to provide a method that reduces the installation or construction loading on the platform thereby permitting a more efficient and lighter structure to be built. A further object of this invention is to provide a method of installing well conductors that is faster and easier to accomplish than the methods now known. These and other objects will become obvious upon further investigation.